This invention relates to a shock sensor which is used, for example, as a collision sensor in an occupant protection device such as an air bag system mounted in a mobile body including an automobile to protect an occupant upon occurrence of a crash or collision of the mobile body. In particular, this invention relates to a shock sensor having a reed switch for detecting impact or acceleration produced upon reception of a shock such as a collision.
A conventional shock sensor of the type described comprises a reed switch, a magnet, and a housing. The magnet is responsive to the impact and moves in a passage defined by an internal surface of the housing along a travelling stroke from a stationary position to a stop position. The magnet is operable to apply a magnetic field to a contact of the reed switch to close the contact when the magnet reaches an area in the vicinity of the reed switch. The area will be referred to as an activating area. The contact is kept closed while the magnet is present in the activating area. Thus, the impact is detected. In the conventional shock sensor, a gap between an outer surface of the magnet and the internal surface of the housing is uniform throughout the travelling stroke. In response to the impact, the magnet uniformly rapidly moves along the travelling stroke because the gap is uniform. In this event, the contact of the reed switch can only be closed for a very short time. It is therefore difficult to obtain an accurate and stabilized electric signal as a detection signal. If such a shock sensor is used as a collision sensor in an occupant protection device such as an air bag system, it is often impossible to supply a processing circuit with a necessary and sufficient signal enough to actuate the occupant protection device upon occurrence of a shock such as a collision.
In the conventional shock sensor, the magnet is brought into contact with a wall surface (stopper) without controlling the movement of the magnet driven by acceleration. This results in increase of a bounding amplitude of the magnet due to bounce at the wall surface and a returning force derived from an elastic force of a spring. Consequently, the contact of the reed switch suffers chattering which often causes an operation error to occur.
In some of the conventional shock sensors, a cylindrical body (holder) is receive din a through hole extending through the magnet and the coil spring. The reed switch is located in the cylindrical body. In this case, the reed switch extends in a direction coincident with a direction of the travelling stroke of the magnet. Accordingly, the shock sensor of this type must be greater in size along an acceleration detecting direction than the reed switch. This results in restriction of the size of the shock sensor.
On the other hand, in the shock sensor, the magnet must be present within a predetermined area or the activating area for a long time so as to increase a closure interval of the contact of the reed switch. In order to provide the closure interval as required, various proposals have been made. For example, a spring constant of the coil spring is selected to be an appropriate value or a frictional coefficient between the magnet and the cylindrical body is increased. However, such proposals alone can not cover various collision modes, for example, a head-on collision and an oblique collision. Thus, it is difficult for the conventional shock sensors to obtain a sufficiently long closure interval.
In some of the conventional shock sensors, use is made of a nonmagnetic block for maintaining the magnet within the activating area. The magnet is retained in the predetermined are of the activating area for a long time by making an inertial force of the nonmagnetic block depend upon variation of the acceleration caused by the collision. With this structure, it is also difficult to provide a sufficiently long closure interval for any one of the various collision modes. In addition, the shock sensor of this type has a structure such that the nonmagnetic block is mounted on a carriage. Accordingly, the movement of the nonmagnetic block completely follows the movement of the carriage. Sometimes, the inertial force of the nonmagnetic block can not be fully utilized due to the movement of the carriage.